Reinforcement of polymer matrices by incorporation of discontinuous or short fiber is well-known. However, the incorporation of discontinuous cellulose fiber into massed polymers is difficult. Intensive prolonged mixing is required to disperse the fiber, and sometimes mix temperatures are generated so high that the composite must be dumped and allowed to cool before further processing. Some cellulose fibers are subject to extensive breakage during mixing. Breakage must be avoided to achieve maximum reinforcement which can only be obtained by preserving the fiber length. It is particularly difficult to obtain high strength fiber-polymer composites using fibers which possess a high degree of fiber interaction, such as unregenerated cellulose fibers, because the fibers tend to cling together in bundles of more than a single fiber and resist dispersion of the individual fibers.
For preparing a composite of polymer matrix reinforced with cellulose fiber, it is recognized that pretreatment of the fiber with material to reduce fiber-to-fiber interaction prior to incorporating it into the matrix aids dispersion and greatly improves the properties of the composite, Boustany and Coran U.S. Pat. No. 3,697,364. The pretreatment appropriately called "predispersion" may be with rubber latex or other substances which reduce fiber-to-fiber interaction. A plasticizer for the cellulose, examples of which are moisture and glycerine, is also present to inhibit fiber breaking on mixing into the polymer matrix. However, the presence of moisture is highly undesirable for some purposes and the presence of volatile substances or substances which plasticize the cellulose to the extent of significantly softening and swelling the fiber structure itself, may have undesirable side effects. Moreover, pretreating the fiber and mixing the treated fiber into the polymer matrix on a factory or semiwork scale are demanding of both time and equipment. The present invention simplifies the preparation of treated fiber and provides an improved polymer treated fiber or concentrate which incorporates into a polymer matrix quickly without excessive fiber breaking and without generating a high mix temperature.